Change History (6)

I don't understand the details about them, but I have a couple of questions about Mercator. First, shouldn't there be false_easting and false_northing attributes, as there are for all other projections? (I think these are just offsets for the map coordinates, aren't they?) Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

I don't understand the details about them, but I have a couple of questions about Mercator. First, shouldn't there be false_easting and false_northing attributes, as there are for all other projections? (I think these are just offsets for the map coordinates, aren't they?)

Yes, I agree. I will add these.

Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

Looks like a mistake on my part since, as you say, Mercator by definition has a latitude of projection at the equator. I will remove the latitude_of_projection_origin attribute.

Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

Looks like a mistake on my part since, as you say, Mercator by definition has a latitude of projection at the equator. I will remove the latitude_of_projection_origin attribute.

You weren't mistaken, Ethan. The Mercator projection can be used with a standard parallel that is not the Equator. In such cases the projection cylinder is secant to the globe rather than tangent to it at the Equator. Of course, one also gets a second standard parallel of equal latitude but opposite sign in the southern hemisphere.

Any map based on such usage would look the same as a standard Mercator map but would have a different scale, with true scale being preserved at all points along the standard parallel rather than the Equator. Which is probably only relevant for navigation charts and the like.

So I think your second Mercator grid mapping parameter does just need to be 'standard_parallel'.